Centrifugal Pump

ABSTRACT

A centrifugal pump includes an impeller rotated about a rotational axis in a rotational direction by a motor and a housing defining a pump chamber therein. The impeller is housed in the pump chamber. The impeller includes a main plate having substantially circular shape and a first surface extending perpendicular to the rotational axis. The main plate is provided with a plurality of blades projecting axially from the first surface and extending radially along the first surface. Each of the blades has a radially inner blade part and a radially outer blade part extending radially outward from the radially inner blade part. Each of the outer blade parts has a front surface that extends from the main plate obliquely either frontward or rearward relative to the rotational direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese patent application serial number 2019-029401, filed Feb. 21, 2019, which is incorporated herein by reference in its entirety for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

This disclosure relates generally to centrifugal pumps.

One type of conventional centrifugal pump includes an impeller rotated by a motor, and a housing defining a pump chamber therein. The impeller is housed in the pump chamber and has a main plate with a circular shape and a plurality of blades radially extending on a top surface of the main plate. The housing includes an inlet port and an outlet port in fluid communication with the pump chamber. The inlet port is formed such that fluid flows into the pump chamber toward a central portion of the top surface of the impeller in a direction perpendicular to the top surface of the main plate of the impeller. The outlet port is formed such that the fluid is discharged from the pump chamber by rotation of the impeller in a tangential direction. The housing of the centrifugal pump includes a partition wall that divides the pump chamber from a flow passage in the outlet port.

BRIEF SUMMARY

In one aspect of this disclosure, a centrifugal pump includes an impeller rotated by a motor in a rotational direction about a central axis and a housing defining a pump chamber therein. The impeller is housed in the pump chamber and includes a main plate having a circular shape and a first surface extending perpendicular to the central axis. The impeller also includes a plurality of blades projecting axially from the first surface of the main plate and extending radially along the first surface. The housing has an inlet port configured to supply fluid into the pump chamber axially toward a center of the first surface of the main plate. The housing also has an outlet port configured to discharge the fluid by rotation of the impeller in a tangential direction relative to the central axis. Each of the blades includes a radially inner blade part and a radially outer blade part that extends radially outward from the radially inner blade part. The radially outer blade part of each blade has a leading surface relative to the rotational direction that extends from the main plate obliquely either frontward or rearward relative to the rotational direction.

In accordance with this aspect, the leading surface of each radially outer blade part extends from the main plate obliquely either frontward or rearward in the rotational direction. Due to this configuration, as each of the radially outer blade parts comes close to the outlet port, the radially outer blade part gradually decreases an opening area defined by the outlet port. Thus, a drastic increase in pressure can be suppressed, thereby offering the potential to decrease vibration and noise caused by pressure pulsations in the centrifugal pump.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiments of the present teaching, reference will now be made to the accompanying drawings.

FIG. 1 is a partial cross-sectional view of a first embodiment of an embodiment of a centrifugal pump in accordance with the principles described herein.

FIG. 2 is a cross-sectional view of the centrifugal pump of FIG. 1 taken along line II-II in FIG. 1.

FIG. 3 is a perspective view of the impeller of FIG. 1.

FIG. 4 is a plan view of the impeller of FIG. 1.

FIG. 5 is a cross-sectional view of the impeller of FIG. 4 taken along line V-V in FIG. 4.

FIG. 6 is a cross-sectional view of the impeller of FIG. 4 taken along line VI-VI in FIG. 4.

FIG. 7 is a cross-sectional view of the impeller of FIG. 4 taken along line VII-VII in FIG. 4.

FIG. 8 is a partial cross-sectional view of a second embodiment of a centrifugal pump in accordance with the principles described herein.

FIG. 9 is a cross-sectional view of the centrifugal pump of FIG. 8 taken along line IX-IX in FIG. 8.

FIG. 10 is a perspective view of the impeller in FIG. 8.

FIG. 11 is a plan view of the impeller of FIG. 8.

FIG. 12 is a cross-sectional view of the impeller of FIG. 11 taken along line MI-MI in FIG. 11.

FIG. 13 is a cross-sectional view of the impeller of FIG. 11 taken along line in FIG. 11.

FIG. 14 is a cross-sectional view of the impeller of FIG. 11 taken along line XIV-XIV in FIG. 11.

FIG. 15 is a cross-sectional view of a radially outer blade part of an embodiment of an impeller in accordance with principles described herein.

FIG. 16 is a cross-sectional view of a radially outer blade part of an embodiment of an impeller in accordance with principles described herein.

DETAILED DESCRIPTION

One conventional centrifugal pump is disclosed in Japanese Laid-Open Patent Publication No. 2008-82268. The centrifugal pump includes blades extending linearly in both a radial direction and an axial direction relative to the central axis of rotation of the impeller. Thus, when each blade comes close to the partition wall dividing the pump chamber from the flow passage in the outlet port, the pressure of fluid therebetween drastically increases. And, just after the blade passes near the partition wall, the pressure of fluid therebetween drastically decreases. Such cyclical pressure changes may generate pressure pulsations, causing vibration and/or noise. Therefore, there has been a need for an improved centrifugal pump.

The following discussion is directed to various exemplary embodiments. However, one skilled in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment.

A first embodiment will be described with reference to FIGS. 1 to 7. In the first embodiment, a centrifugal pump 10 is used as a purge pump mounted on a vehicle, such as an automobile. The purge pump is configured to compensate for a shortage of purge gas flowing from a canister to an air intake passage of an internal combustion engine. Exemplary coordinate axes are shown in FIGS. 1 and 2 for purposes of further explanation and clarity. However, these coordinate axes do not limit the orientation or the mounting direction of the centrifugal pump 10 on the vehicle.

As shown in FIG. 1, the centrifugal pump 10 of the first embodiment includes a housing 11 having a substantially a hollow cylindrical shape. The centrifugal pump 10 also includes a pump section 12 at an upper portion of the housing 11 and a motor section 14 positioned below the pump section 12.

The motor section 14 includes a brushless motor and a rotational shaft 15 extending in the vertical direction. As will be described in more detail below, the rotational shaft 15 has a central axis (also extending in the vertical direction) that defines a rotational axis of an impeller 30 of centrifugal pump 10. The motor section 14 may also be referred to herein as a “motor.”

The housing 11 defines a pump chamber 17 in an upper portion thereof. The pump chamber 17 may have a hollow short cylindrical shape and be coaxially aligned with the rotational shaft 15 of the motor section 14. The housing 11 is divided in the axial direction into an upper housing member 11 a and a lower housing member 11 b. The pump chamber 17 is formed by the upper housing member 11 a and the lower housing member 11 b being coupled together. The pump chamber 17 has a volute part 17 a formed at an outer circumference thereof. The rotational shaft 15 of the motor section 14 penetrates the lower housing member 11 b and protrudes axially into the pump chamber 17.

The upper housing member 11 a includes an inlet port 20 that may have a hollow cylindrical shape extending upward from a top surface of the upper housing member 11 a. The inlet port 20 is coaxially aligned with the pump chamber 17 and the rotational shaft 15. The inlet port 20 defines an inflow passage 20 a therein. The inflow passage 20 a provides fluid communication between the inside and the outside of the pump chamber 17.

As shown in FIG. 2, the upper housing member 11 a of the housing 11 includes an outlet port 22 that may have a hollow cylindrical shape extending leftward in FIGS. 1 and 2 from a front portion of the upper housing member 11 a. In the plan view, the outlet port 22 extends tangentially from the radially outer perimeter of the pump chamber 17. The outlet port 22 defines an outflow passage 22 a therein. The outflow passage 22 a provides fluid communication between the inside and the outside of the pump chamber 17. The upper housing member 11 a of the housing 11 includes a partition wall 24 separating and dividing the outflow passage 22 a from the volute part 17 a of the pump chamber 17.

The outlet port 22 includes an upstream open end 22 b positioned at or proximal the volute part 17 a of the pump chamber 17. As shown in FIG. 1, in this embodiment, the upstream open end 22 b has an elliptical shape with a major axis extending in the horizontal direction. However, in other embodiments, the upstream open end 22 b may have another shape, such as a rectangular shape.

The impeller 30 is rotatably disposed in the pump chamber 17. The impeller 30 is coaxially aligned with the rotational shaft 15 and is fixably mounted to an upper end of the rotational shaft 15 of the motor section 14. Thus, when the rotational shaft 15 rotates about its central, rotational axis, the impeller 30 simultaneously rotates in a rotational direction R about the central, rotational axis of the shaft 15. In FIG. 2, rotational direction R is in a clockwise direction. The impeller 30 may be made from a resin material. As shown in FIG. 1, the inlet port 20 generally faces a central portion of a blade side of the impeller 30 (i.e. a central portion of the top surface side of the impeller 30 in this embodiment).

When the motor section 14 is driven using electricity supplied from an external power source, the impeller 30 is rotated together with the rotational shaft 15, so that fluid, in this embodiment purge gas, is suctioned into the pump chamber 17 via the inflow passage 20 a of the inlet port 20. The purge gas is pressurized and then discharged into the outflow passage 22 a via the upstream open end 22 b of the outlet port 22 due to the rotation of the impeller 30. In this manner, the centrifugal pump 10 pumps the purge gas. When the purge gas is discharged into the upstream open end 22 b of the outlet port 22 via rotation of the impeller 30, a radially outer end of at least one of the blades 36 of the impeller 30 passes close to the partition wall 24.

As shown in FIG. 3, the impeller 30 includes a main plate 32, a boss 34, and a plurality of blades 36 mounted to main plate 32. In this embodiment, twelve blades 36 are provided. The main plate 32 has substantially a circular plate shape extending perpendicular to the rotational shaft 15 and the central axis thereof. The main plate 32 includes a convex part 32 a on an upper surface thereof. The convex part 32 a has a truncated cone shape surrounding the boss 34. The upper surface of the main plate 32 may also be referred to herein as “a first surface” of the main plate 32. The boss 34 is formed in a hollow cylindrical shape having a closed top and an open bottom. The boss 34 is coaxially disposed at a central portion of the upper surface of the main plate 32. As shown in FIG. 2, the rotational shaft 15 of the motor section 14 is fixedly inserted into the boss 34. As shown in FIG. 3, each of the blades 36 protrudes axially upward from the upper surface of the main plate 32 and extends radially along the upper surface of the main plate 32. Each of the blades generally has an elongated, generally rectangular plate shape oriented in the radial direction.

In this embodiment, all of the blades 36 may have the same shape as each other. Accordingly, only one of the blades 36 will be described for convenience of explanation with the understanding the other blades 36 are the same. As shown in FIG. 4, a radially inner end of the blade 36 is coupled to the boss 34 and deviates forward from a virtual line, which extends between a radially outer end of the blade 36 and a rotational center 30 c of the impeller 30 defined by the rotational axis of shaft 15. In the plan view of the impeller 30, an upper edge of the blade 36 gently curves rearward relative to the rotational direction of the impeller 30 from the radially inner end toward the radially outer end of the blade 36. A lower edge of the blade 36 may also be referred to herein as “a main plate side part” of the blade 26, and the upper edge of the blade 36 may also be referred to herein as “an opposite side part” of the blade 26. The plan view illustrated by FIG. 4 corresponds to “a plan view of the impeller” in this disclosure.

The blade 36 is divided into a radially inner blade part 37 and a radially outer blade part 38. The radially inner blade part 37 extends radially from the boss 34 to the corresponding radially outer blade part 38, and the radially outer blade part 38 extends radially outward from the radially inner blade part 37. In FIG. 4 depicting a plan view of the impeller 30, a radial vector N of the main plate 32 passes through both the rotation center 30 c of the impeller 30 and a connection part between the inner blade part 37 and the outer blade part 38. The inner blade part 37 is positioned in front of the radial vector N relative to the rotational direction R of the impeller 30. A radially outer end of the inner blade part 37 is contiguous with and coupled to a radially inner end of the outer blade part 38. The blade 36 includes a front surface facing forward relative to the direction of rotation R and a rear surface facing rearward relative to the direction of rotational R. The radial vector N may pass through the front surface of the blade 36 at the radially inner end of the outer blade part 38.

As shown in FIG. 5, the inner blade part 37 of the blade 36 includes a front surface 37 a and a rear surface 37 b, each of which extends perpendicular to the upper surface of the main plate 32 (i.e., axially relative to the rotational axis). In this disclosure, the term “perpendicular” includes both perpendicular and substantially perpendicular.

As shown in FIGS. 6 and 7, a front surface 38 a of the outer blade part 38 of the blade 36 extends from the upper surface of the main plate 32 obliquely rearward relative to the rotational direction of the impeller 30. For instance, an angle formed between the upper surface of the main plate 32 and a lower end of the front surface 38 a of the outer blade part 38 may be oblique. As shown in FIGS. 3 and 4, the front surface 37 a of the inner blade part 37 and the front surface 38 a of the outer blade part 38 are contiguous with each other. As shown in FIGS. 6 and 7, in the cross-sectional view perpendicular to a longitudinal direction of the outer blade part 38, the front surface 38 a extends linearly between its upper end and a lower end in this embodiment. However, in other embodiments, the front surface 38 a may be gently curved in a concave or convex manner in some embodiments.

In the cross-sectional view perpendicular to the longitudinal direction of the outer blade part 38, the front surface 38 a of the outer blade part 38 defines an included angle 380 with a straight line L that extending perpendicular to the upper surface of the main plate 32 and passing through the upper end of the front surface 38 a. That is, the included angle 380 is formed by the front surface 38 a and the straight line L and is acute. The included angle 380 between the front surface 38 a and the straight line L gradually increases from the radially inner end toward the radially outer end of the outer blade part 38. The direction along which the straight line L is disposed may also be referred to herein as an “axis disposed in the first direction.”.

As shown in FIG. 4, in the plan view of the impeller 30, a lower half of the front surface 38 a of the radially outer end of the outer blade part 38 is positioned in front of the radial vector N relative to the rotational direction R, and an upper half of this portion of the front surface 38 a is positioned rearward of the radial vector N relative to the rotational direction R. Such a configuration is also illustrated in FIGS. 6 and 7, in which a dashed line N′ shows a straight line extending vertically and perpendicular to the radial vector N. As shown in FIGS. 3 and 4, a radially outer end surface of the outer blade part 38 is flush with an outer periphery of the main plate 32. A lower half of the front surface 38 a may also be referred to herein as “a main plate side part.”, and the upper half of the front surface 38 a may also be referred to herein as “an opposite side part”. The direction along which the radial vector N is disposed may also be referred to herein as a “normal axis.”

As shown in FIGS. 6 and 7, a thickness 38 t of the outer blade part 38 in the rotational direction R gradually increases from the upper end toward the lower end of the outer blade part 38. The rear surface 38 b of the outer blade part 38 extends perpendicular to the upper surface of the main plate 32. As shown in FIG. 3, the rear surface 38 b of the outer blade part 38 is contiguous with the rear surface 37 b of the inner blade part 37. As shown in FIG. 5, a thickness 37 t of the inner blade part 37 in the rotational direction R is constant between an upper end and a lower end of the inner blade part 37. In this disclosure, the term “constant” includes both constant and substantially constant.

In accordance with the first embodiment, the front surface 38 a of the outer blade part 38 of each blade 36 extends from the upper surface of the main plate 32 obliquely rearward relative to the rotational direction R Thus, as the radially outer end of each outer blade part 38 comes close to the partition wall 24, the radially outer end gradually decreases the opening area of the upstream open end 22 b of the outlet port 22. Accordingly, a drastic increase in pressure can be suppressed, so that vibration and noise caused by pressure pulsations in the centrifugal pump 10 can be decreased.

The included angle 380 between the front surface 38 a and the straight line L gradually increases from the radially inner end toward the radially outer end of the outer blade part 38. Such configuration of the front surface 38 a can smoothen the fluid flow outward in the radial direction of the impeller 30.

As shown in FIG. 4, in the plane view of the impeller 30, the lower half of the front surface 38 a of the radially outer end of each outer blade part 38 is positioned in front of the radial vector N in the rotational direction R. Thus, the pressurizing performance of the centrifugal pump 10 is increased, so that the number of revolution required for achieving a predetermined pressure can be decreased. In addition, the upper half of the front surface 38 a of the radially outer end of each outer blade part 38 is positioned rearward of the radial vector N relative to the rotational direction R. Such configuration can smoothen the fluid flow outward in the radial direction of the impeller 30 so as to decrease a load torque on the impeller 30. Accordingly, a decrease in both the number of revolution of the centrifugal pump 10 and the load torque on the impeller 30 is achieved, thereby improving pump efficiency.

The thickness 38 t of each outer blade part 38 gradually increases from the upper end toward the lower end of the outer blade part 38. Thus, the rigidity of the outer blade part 38 can be improved, thereby further reducing vibration and noise.

The rear surface 38 b of each outer blade part 38 extends upward from the main plate 32 in a direction perpendicular to the upper surface of the main plate 32. So, when the impeller 30 is made from a resin material using molds, at least one of the molds, which shapes the corresponding rear surface 38 b, can be removed in the axial direction of the impeller 30. Accordingly, the molds can be simplified, thereby improving manufacturing of the impeller 30.

A second embodiment will be described with reference to FIGS. 8 to 14. The second embodiment is substantially the same as the first embodiment described above with some differences regarding the outer blade parts 38 of the impeller 30. Thus, while the differences will be described, similar configurations will not be described in the interest of conciseness.

As shown in FIGS. 9, 10, and 11, each of the blades 36 includes a radially outer blade part 138 extending radially outward from the radial outer end of the inner blade part 37. In this embodiment, all of the outer blade parts 138 have substantially the same shape as each other. Accordingly, only one radially outer blade part 138 will be described with the understanding the other radially outer blade parts 138 are the same. The outer blade part 138 includes a front surface 138 a extending from the main plate 32 of the impeller 30 obliquely frontward relative to the rotational direction R of the impeller 30. As shown in FIG. 12, the inner blade part 37 of the second embodiment has the same shape as that of the first embodiment.

As shown in FIGS. 13 and 14, in the cross-sectional view perpendicular to the longitudinal direction of the outer blade part 138, an included angle 1380 of the front surface 138 a, which is defined by the front surface 138 a and a straight line L that extends perpendicular to the upper surface of the main plate 32 and passes through the upper end of the front surface 138 a, is acute. The included angle 1380 formed between the front surface 138 a and the straight line L gradually increases from the radially inner end toward the radially outer end of the outer blade part 138.

In FIG. 11, one radial vector N of the boss 34 is shown. The radial vector N passes through both the rotation center 30 c of the impeller 30 and a connection part between the inner blade part 37 and the outer blade part 138. In the plan view of the impeller 30, an upper half of the front surface 138 a of the radially outer end of the outer blade part 138 is positioned in front of the radial vector N relative to the rotational direction R of the impeller 30, and a lower half of the front surface 138 a of the radially outer end of the outer blade part 138 is positioned rearward of the radial vector N relative to the rotational direction R.

As shown in FIGS. 13 and 14, the thickness 138 t of the outer blade part 138 is constant between an upper end and a lower end of the outer blade part 138. In addition, the thickness 138 t is constant between the radially inner end and the radially outer end of the outer blade part 138, i.e. over the longitudinal whole length of the outer blade part 138.

As shown in FIGS. 10 and 11, an outer periphery of the main plate 32 is concave and curved radially inward between each pair of adjacent blades 36, such that the radial length of the main plate 32 gradually decreases from a front of each blade 36 toward the adjacent blade 36 in a direction opposite to the rotational direction R. Due to this configuration, the main plate 32 defines a plurality of circumferentially-spaced openings 140 along the outer periphery thereof such that each of the front surfaces 138 a of the outer blade part 138 faces a corresponding opening 140.

In accordance with the second embodiment, the front surface 138 a of the outer blade part 138 of each blade 36 of the impeller 30 extends from the upper surface of the main plate 32 obliquely frontward relative to the rotational direction R of the impeller 30. Thus, as the radially outer end of each outer blade part 138 comes close to the partition wall 24 of the housing 11, which is illustrated in FIGS. 8 and 9, the radially outer end gradually decreases the opening area of the upstream open end 22 b of the outlet port 22. Accordingly, a drastic increase in pressure can be prevented, thereby decreasing vibration and noise caused by pressure pulsations of the centrifugal pump 10.

The included angle 1380 formed between the front surface 138 a and the straight line L gradually increases from the radially inner end toward the radially outer end of the outer blade part 138. Such configuration of the outer blade part 138 can smoothen the fluid flow outward in the radial direction of the impeller 30.

In the plan view of the impeller 30, the upper half of the front surface 138 a of the radially outer end of each outer blade part 138 is positioned in front of the radial vector N in the rotational direction R. Thus, pressurizing performance of the centrifugal pump 10 is increased, so that the number of revolution required for achieving a predetermined pressure can be decreased. In addition, the lower half of the front surface 138 a of the radially outer end of each outer blade part 138 is positioned rearward of the radial vector N in the rotational direction R. Such configuration can smoothen the fluid flow outward in the radial direction of the impeller 30 so as to decrease a load torque on the impeller 30. Accordingly, a decrease in both the number of revolution of the centrifugal pump 10 and the load torque on the impeller 30 is achieved, thereby improving pump efficiency.

The main plate 32 defines the openings 140 that the front surfaces 138 a of the outer blade parts 138 face. So, when the impeller 30 is made from a resin material using molds, at least one of the molds, shaping the corresponding front surface 138 a, can be removed in the axial direction of the impeller 30. Accordingly, the mold can be simplified, thereby improving manufacturing of the impeller 30.

A third embodiment will be described with reference to FIG. 15. The third embodiment is substantially the same as the second embodiment described above with some modifications regarding the outer blade parts 138 of the impeller 30. Thus, while the differences will be described, similar configurations will not be described in the interest of conciseness.

As shown in FIG. 15, a radially outer blade part 238 of each blade 36 includes a front surface 238 a formed with a similar configuration as the front surface 138 a of the second embodiment. However, the outer blade part 238 is also shaped such that its thickness 238 f in the rotational direction R gradually increases from the upper end toward the lower end of the outer blade part 238. Thus, the rigidity of the outer blade parts 238 can be increased, thereby reducing vibration and breakage of the outer blade parts 238.

A fourth embodiment will be described with reference to FIG. 16. The fourth embodiment is substantially the same as the first embodiment described above with some modifications regarding the outer blade parts 38 of the impeller 30. Thus, while the differences will be described, similar configurations will not be described in the interest of conciseness.

As shown in FIG. 16, each radially outer blade part 338 of the impeller 30 includes a front surface 338 a formed with a similar configuration as the front surface 38 a of the first embodiment. However, the thickness 338 t of the outer blade part 338 is constant between an upper end and a lower end of the outer blade part 338. That is, a rear surface 338 a of each outer blade part 338 faces obliquely downward relative to the rotational direction R.

Although not shown in the drawings, an outer periphery of a main plate 332 is concave and curved radially inward between each pair of circumferentially adjacent blades 36, such that the radial length of the main plate 332 gradually increases from a front of one blade 36 toward an adjacent blade 36. Due to this configuration, the main plate 332 of the impeller 30 defines a plurality of circumferentially-spaced openings 340 along the outer periphery thereof, such that each of the rear surfaces 338 b of the outer blade part 338 faces the corresponding opening 340. Thus, when the impeller 30 is made from a resin material using molds, at least one of the molds, shaping the corresponding rear surface 338 b, can be removed in the axial direction of the impeller 30. Accordingly, the molds can be simplified, thereby improving manufacturing of the impeller 30.

As mentioned above, the apparatus and methods disclosed in this application are not limited to the above-described embodiments. For example, the centrifugal pump 10 may be used for pumping various fluid, such as air, other than a purge gas. The brushless motor of the motor section 14 may be replaced with a brushed motor. The centrifugal pump 10 may be comprised of the motor section 14 only such that the rotational shaft 15 is rotated by a driving source that is provided outside the centrifugal pump 10. The impeller 30 may be made from a metal material. 

What is claimed is:
 1. A centrifugal pump, comprising: an impeller configured to be rotated in a rotational direction about a rotational axis by a motor, wherein the impeller includes a main plate and a plurality of blades, wherein the main plate has a substantially circular shape and a first surface, wherein each of the blades extends axially from the first surface of the main plate and radially along the first surface of the main plate; and a housing defining a pump chamber that houses the impeller therein, wherein the housing includes an inlet port configured to supply a fluid into the pump chamber axially toward a center of the first surface of the main plate and an outlet port configured to discharge the fluid in a tangential direction from the pump chamber, wherein: each of the blades has a radially inner blade part and a radially outer blade part extending radially outward from the radially inner blade part, and each of the radially outer blade parts has a front surface that extends from the main plate obliquely rearward relative to the rotational direction.
 2. The centrifugal pump of claim 1, wherein: an angle of the front surface of one of the radially outer blade parts relative to a straight line extending in parallel with the rotational axis is acute and continuously increases moving radially outward.
 3. The centrifugal pump of claim 1, wherein: one of the radially outer blade parts includes a radially outer end that is divided into a main plate side part adjacent to the main plate and an opposite side part positioned away from the main plate, a reference line passes radially through both the rotational axis of the impeller and a radially inner end of the one of the radially outer blade parts, the main plate side part is positioned in front of the reference line relative to the rotational direction, and the opposite side part is positioned rearward of the reference line relative to the rotational direction.
 4. The centrifugal pump of claim 1, wherein: a thickness in the rotational direction of each of the radially outer blade parts continuously increases moving axially toward the first surface.
 5. The centrifugal pump of claim 4, wherein: each of the radially outer blade parts has a rear surface extending perpendicular to the main plate or extends obliquely rearward relative to the rotational direction.
 6. A centrifugal pump, comprising: an impeller configured to be rotated about a rotational axis in a rotational direction by a motor, wherein the impeller includes a main plate and a plurality of blades, wherein the main plate has a substantially circular shape and a first surface, wherein each of the blades projects axially from the first surface of the main plate and extends radially along the first surface of the main plate; and a housing defining a pump chamber that houses the impeller therein, wherein the housing includes an inlet port configured to supply a fluid into the pump chamber axially toward a center of the first surface of the main plate and an outlet port configured to discharge the fluid from the pump chamber in a tangential direction, wherein: each of the blades has a radially inner blade part and a radially outer blade part that extends radially outward from the radially inner blade part, and each of the radially outer blade parts has a front surface that extends from the main plate obliquely frontward relative to the rotational direction.
 7. The centrifugal pump of claim 6, wherein: an angle of the front surface of one of the radially outer blade parts relative to a straight line extending in parallel with the rotational axis is acute and continuously increases moving radially outward.
 8. The centrifugal pump of claim 6, wherein: one of the radially outer blade parts includes a radially outer end having a main plate side part adjacent to the main plate and an opposite side part positioned away from the main plate, a reference line passes radially through both the rotational axis of the impeller and a radially inner end of the one of the radially outer blade parts, the main plate side part is positioned rearward of the reference line relative to the rotational direction, and the opposite side part is positioned in front of the reference line relative to the rotational direction.
 9. The centrifugal pump of claim 6, wherein: a thickness in the rotational direction of each of the radially outer blade parts continuously increases moving axially toward the first surface.
 10. The centrifugal pump of claim 9, wherein: an outer periphery of the main plate is curved radially inward between a pair of circumferentially adjacent blades to define an opening along the outer periphery of the main plate, and the front surface of the radially outer blade part of a rear one of the pair of circumferentially adjacent blades relative to the rotational direction faces the opening. 